The present invention relates to a device comprising: a terminal for connection of the device to a power source; a rectifier connected to the terminal; a half-bridge circuit connected to the rectifier in order to generate an a.c. switching voltage and having a half-bridge point; a control circuit for controlling the half-bridge circuit; a power supply for deriving a supply voltage for the control circuit from the a.c. switching voltage supplied by the half-bridge circuit, said power supply comprising a charging capacitor connected to said half-bridge point, and a load circuit connected to the half-bridge point.
Such a device is known from U.S. Pat. No. 5,705,894, where the power supply comprises essentially a pair of diodes, where the charging capacitor is connected, on the one hand, to the node between the pair of diodes and, on the other hand, directly to the output of the half-bridge circuit. The half-bridge circuit, together with the control circuit, forms a half-bridge resonant converter for driving a load.
The control circuit of the known device includes means for selectively accepting the supply voltage by simply short-circuiting the power supply when the applied supply voltage is not within a predetermined range.
A disadvantage of the known device is that dissipation especially in at least one of the pair of diodes of the power supply, is high, especially when high rectified voltages from the rectifier are supplied to the half-bridge circuit.
According to the invention, in order to obviate or at least mitigate the above-mentioned problems of the known device, a device is provided which is characterized in that the charging capacitor is connected to the half-bridge point through an impedance. The present invention is based on the insight that an impedance can stabilize the current through the above-mentioned one of the pair of diodes, which current will therefore remain practically constant, whatever the rectified voltage from the rectifier. Thus, dissipation is reduced and a more economical device is provided.
Preferably, the impedance connecting the charging capacitor to the half-bridge point comprises a coil in series with the charging capacitor. This is a very elegant and simple embodiment of the present invention.
The load circuit can be connected to the half-bridge point through a transformer, which transformer has its primary side connected to the half-bridge point. Such a transformer has the advantage that a power level suitable for the load circuit can be obtained. The load circuit can be connected to the half-bridge point through a connecting impedance, which is preferably a coil.
In a configuration where the load is connected to the half-bridge point through a connecting impedance, the charging capacitor is preferably connected to the connecting impedance at the side of the connecting impedance remote from the half-bridge point. Thus, the impedance connected between the half-bridge point and the load circuit at the same time forms the impedance through which the charging capacitor is connected to the half-bridge point. Thus, an additional component to form the impedance between the charging capacitor and the half-bridge point is no longer necessary, and the present invention is realized without having to add components to the device.
The present invention is especially advantageous when the device is portable and its terminal can be connected to a number of power sources with different power levels. Such devices can be portable razors, battery chargers, for example for a mobile phone etc. When travelling, a user is confronted with many power levels of local power sources. Such power levels range between 100 V and 230 V approximately. The rectified voltages provided by the rectifier therefore vary between essentially 100 and 400 V. Without the features of the present invention the dissipation of said one of the pair of diodes in the power supply at 400 V can be eight times as high as the dissipation thereof at 100 V. Dimensioning this diode for worst-case conditions (400 V) would result in such large diode that it could no longer be used in the portable devices, which are preferably as small as possible. If this dissipating diode would not be dimensioned for worst-case conditions (400 V) in order to save room, the dissipating diode would overheat when subjected to such worst-case conditions. According to the present invention this problem is effectively overcome.